Blake T. Ostler argues that the Book of Mormon, while an ancient text, has modern expansions made by Joseph on a revelatory level; this explains many of the purported anachronisms in the book.
Blake T. Ostler, "The Book of Mormon as a Modern Expansion of an Ancient Source," Dialogue: A Journal of Mormon Thought (Spring 1987): 50-125
Evan a casual reference to studies treating the Book of Mormon reveals a range of divergent explanations of its origins. At one extreme are those who are skeptical of the book's claims to antiquity who generally conclude that it is a pious fraud, written by Joseph Smith from information available in his immediate environment. At the other extreme are those who accept the book as scripture and suggest that it can be explained exclusively by reference to ancient sources either not available to Joseph Smith or available only if he were capable of the most recondite research and near-genius ability in comparative literature and ancient studies.
It is my purpose to demonstrate that both extremes are too limited and to offer a theory of the Book of Mormon as Joseph Smith's expansion of an ancient work by building on the work of earlier prophets to answer the nagging problems of his day. In so doing, he provided unrestricted and authoritative commentary, interpretation, explanation, and clarifications based on insights from the ancient Book of Mormon text and the King James Bible (KJV). The result is a modern world view and theological understanding superimposed on the Book of Mormon text from the plates.
. . .
CONCLUSION
This essay has attempted to identify and define some expansions of the Book of Mormon and to demonstrate the value of such a model as an explanation of the book. The expansion model requires coming to grips with larger issues concerning the historicity of scripture and the plausibility of revelation as a partial explanation. Evidences concerning the historicity of the Book of Mormon certainly will never be explained to the satisfaction of all, but a universally acceptable proof is not necessary to show that many of our common assumptions about scripture prevent an adequate interpretation of scriptures and their historicity.
The conclusion that the Book of Mormon is pious fraud derived from nineteenth-century influences does not logically follow from the observation that it contains KJV quotations and is expressed in terms of a nineteenth-century world view. Nor does it follow that doctrinal developments cast doubts on whether earlier expressions reflected an authentic encounter with God. All expressions of revelation must be communicated within their author's framework of thought, a framework limited by its assumptions. Nor does it follow that if the book derives from the revelation of an ancient source it must be explained exclusively in ancient terms. Fundamentalist views of revelation and scripture that give rise to such assumptions are grossly inadequate.